A 45 year old female is brought to the ER with lightheadedness and palpitations of acute onset. She has also experienced fatigability, weight loss and insomnia lately. She denies smoking cigarettes and drinking alcohol. She is currently not taking any medication. Her blood pressure is 115/80 mmHg and heart rate is 120/min, regular. Physical examination reveals fine tremor when her hands are outstretched and lid lag. ECG shows atrial fibrillation. What is the next best step in the management of this patient?
C. Immediate cardioversion